Administration of testosterone has been part of clinical practice for over 50 years. It is an efficacious and rational treatment for hypogonadal men. Several methods of testosterone replacement have been evaluated. Most widely used is intramuscular injections of testosterone, but this strategy has been hampered by ""peaks and troughs"" in blood levels of the hormone following the injection. Oral administration, unfortunately, has been associated with a significant incidence of transient liver abnormalities, including jaundice. Recently, transdermal testosterone patch systems have become available in several countries, and in many hypogonadal men, these transdermal systems can mimic the diurnal variations in testosterone levels that are seen in healthy men. These studies are designed to test the pharmacokinetics of testosterone delivered through transdermal patches in men with hypogonadism from a variety of causes.